This invention relates to a power electronic converter for use in high voltage direct current (HVDC) power transmission and reactive power compensation.
In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometer of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.
The conversion of AC to DC power is also utilized in power transmission networks where it is necessary to interconnect AC networks operating at different frequencies.
In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion, and one such form of converter is a voltage source converter (VSC).
An example of a voltage source converter is the use of six-switch (two-level) and three-level multilevel converter topologies with insulated gate bipolar transistors (IGBT) 20, as shown in FIGS. 1a and 1b. The IGBT devices 20 are connected and switched together in series to enable high power ratings of 10's to 100's of MW to be realized.
This conventional approach requires a complex and active IGBT drive, and may require large passive snubber components to ensure that the high voltage across the series strings of IGBT devices 20 shares properly during converter switching. In addition, the IGBT devices 20 need to switch on and off several times at high voltage over each cycle of the AC supply frequency to control the harmonic currents being fed to the AC network 24.
Another example of a voltage source converter lies in a multilevel converter arrangement which is shown in FIG. 2. In the conventional multilevel converter, converter bridges or cells 26 are connected in series, each cell 26 being switched at a different time. Each cell 26 includes a pair of semiconductor switches 28 connected in parallel with a capacitor 30 in a half bridge arrangement to define a 2-quadrant unipolar module that can develop zero or positive voltage and can conduct current in both directions. The conventional multilevel converter arrangement eliminates the problems associated with the direct switching of series connected IGBT devices, because the individual bridge cells 26 do not switch simultaneously and converter voltage steps are comparatively small.
During operation of HVDC power transmission networks, voltage source converters may however be vulnerable to DC side faults that present a short circuit with low impedance across the DC power transmission lines or cables. Such faults can occur due to damage or breakdown of insulation, movement of conductors or other accidental bridging between conductors by a foreign object.
The presence of low impedance across the DC power transmission lines or cables is detrimental to a voltage source converter because it can cause current flowing in the voltage source converter to increase to a fault current level many times above its original value. In circumstances where the voltage source converter was only designed to tolerate levels of current below the level of the fault current, such a high fault current damages components of the voltage source converter.
Conventionally, in order to reduce the risk posed by a short circuit to a device, one or more switches would be opened to switch the device out of circuit. However the switching elements of voltage source converters, such as the voltage source converter shown in FIG. 1a, typically include anti-parallel diodes 22 that remain in conduction when the insulated gate bipolar transistors 20 are opened. Consequently, even when the insulated gate bipolar transistors 20 are opened, the diodes 22 allow the fault current 32 arising from a short circuit 34 in a DC network 36 connected to the voltage source converter to flow continuously through the converter, as shown in FIG. 3.
Another option for reducing the risk posed to a voltage source converter by a short circuit is to design the voltage source converter to tolerate the resultant fault current so that there is sufficient time to detect the fault and extinguish the current by opening a circuit breaker on the other, AC side of the voltage source converter.
However the fault current arising from a short circuit in a DC network connected to the voltage source converter is typically many times greater than the rated value of the converter. In order to increase the tolerance of the voltage source converter, either the size and capacity of conducting converter diodes must be increased, several converter diodes must be connected in parallel or a fast-acting bypass device must be incorporated that is capable of carrying the high fault current. In any case, whichever option is pursued, additional inductive components are almost certainly required to limit the high fault current and the increase in components leads to an increase in converter size and weight. This in turn leads to an increase in the size and area of the associated HVDC converter station.
In addition, opening a circuit breaker on the opposite, non-faulty side of the voltage source converter is disadvantageous because it disconnects the other network from the HVDC power transmission network. Consequently after the fault is repaired, the converter station must go through a start-up sequence and a series of checks before the other network can be reconnected to the HVDC power transmission network. This leads to a prolonged interruption of power flow and therefore non-availability of the power transmission scheme to those dependent on the scheme for electrical power supply.
A further option is to open a circuit breaker on the DC side of the voltage source converter to allow the fault in the DC network to be isolated and repaired. However, the non-zero direct current flowing in the voltage source converter results in the formation of a sustained power arc when conventional mechanical circuit breaking contacts are used. It is therefore necessary to use expensive, specialised DC circuit breaking equipment to interrupt the DC side fault current, which leads to an increase in converter size, weight and cost.